The present disclosure relates to image forming apparatuses such as copiers, printers, and facsimile machines as well as multifunction peripherals having their functions integrated together, and more particularly to image forming apparatuses provided with a developing device that is supplied with two-component developer containing toner and carrier while discharging excess developer.
In an image forming apparatus, a latent image is formed on an image carrier such as a photosensitive member, and the latent image is developed by a developing device into a toner image so as to be visible. Some developing devices adopts two-component development using two-component developer. In this type of developing device, two-component developer containing toner and carrier is contained inside a developing container, a developing roller is arranged to feed the developer to the image carrier, and a stirring member is arranged to feed, while stirring and transporting, the developer inside the developing container to the developing roller.
With this developing device, as development operation proceeds, while toner is consumed, the carrier is not consumed but remains inside the developing container. Thus, the carrier stirred with the toner inside the developing container deteriorates as it is stirred repeatedly, and accordingly the carrier gradually loses its ability to electrically charge the toner. As an improvement, a developing device has been proposed in which the developing container is supplied with developer containing carrier while excess developer is discharged with a view to suppressing degradation in chargeability.
For example, a developing device is known in which a stirring member is provided to stir and transport developer inside a developing container, a developer discharge port is provided downstream with respect to the transport direction, and a restricting member is provided between the stirring member and the developer discharge port, the restricting member being formed in a helical shape spiraling in the opposite direction to a helical blade on the stirring member. With this structure, when developer is supplied into the developing container, as the stirring member rotates, the developer is, while being stirred, transported to the downstream side in the transport direction. As the restricting member rotates in the same direction as the stirring member, a transporting force acting in the direction opposite to the direction of transport by the stirring member is applied to the developer. This transporting force acting in the opposite direction causes the developer to stop and bulge on the downstream side in the transport direction, and thus excess developer crosses over the restricting member and moves to the developer discharge port so as to be discharged out of the developing container.
In the developing device described above, so long as the stirring member rotates at a constant speed, a constant amount of developer is discharged through the developer discharge port, and thus the amount of developer inside the developing container is stable. However, in some developing devices, the rotation speed of the stirring member is switchable, in which case, as the rotation speed of the stirring member is switched, the amount of developer discharged through the developer discharge port varies with the rotation speed.
For example, the image formation speed is switched according to type of paper, such as thickness, and print mode, such as high-gloss printing. Also, in a case where a single type of developing device with particular specifications is shared among a plurality of types of image forming apparatuses, for example, the printing speed for an A4-size recording medium varies from one model of image forming apparatus to the other, and thus different image formation speeds need to be set to suit the printing speeds of different models. As the image formation speed is so switched, the rotation speed of the stirring member is accordingly switched, and thus the amount of developer discharged through the developer discharge port varies with the rotation speed of the stirring member.